suppose I have a MongDB record like below:
{
name:"name",
streams: [
{user:"user0", name:"name0", locked:true},
{user:"user1", name:"name1", locked:true},
{user:"user2", name:"name2", locked:false}
}
}
I want to find all records that have user0 and name0 in the streams field, but I don't care about the locked field
find({streams:{user:"user0", name:"name0"}}) doesn't work, since the locked field is not specified.
Thank You,
Gary
You are looking for the $elemMatch operator which allows you to select the fields from a sub-document in an array that match your conditions:
db.collection.find({
"streams": { "$elemMatch": { "user": "user0", "name": "name0"} }
})
Take some time to go through the Query Operators in the manual. There are lots of useful operations there.
Related
select records using aggregate:
db.getCollection('stock_records').aggregate(
[
{
"$project": {
"info.created_date": 1,
"info.store_id": 1,
"info.store_name": 1,
"_id": 1
}
},
{
"$match": {
"$and": [
{
"info.store_id": "563dcf3465512285781608802a"
},
{
"info.created_date": {
$gt: ISODate("2021-07-18T21:07:42.313+00:00")
}
}
]
}
}
])
select records using find:
db.getCollection('stock_records').find(
{
'info.store_id':'563dcf3465512285781608802a',
'info.created_date':{ $gt:ISODate('2021-07-18T21:07:42.313+00:00')}
})
What is difference between these queries and which is best for select by id and date condition?
I think your question should be rephrased to "what's the difference between find and aggregate".
Before I dive into that I will say that both commands are similar and will perform generally the same at scale. If you want specific differences is that you did not add a project option to your find query so it will return the full document.
Regarding which is better, generally speaking unless you need a specific aggregation operator it's best to use find instead, it performs better
Now why is the aggregation framework performance "worse"? it's simple. it just does "more".
Any pipeline stage needs aggregation to fetch the BSON for the document then convert them to internal objects in the pipeline for processing - then at the end of the pipeline they are converted back to BSON and sent to the client.
This, especially for large queries has a very significant overhead compared to a find where the BSON is just sent back to the client.
Because of this, if you could execute your aggregation as a find query, you should.
Aggregation is slower than find.
In your example, Aggregation
In the first stage, you are returning all the documents with projected fields
For example, if your collection has 1000 documents, you are returning all 1000 documents each having specified projection fields. This will impact the performance of your query.
Now in the second stage, You are filtering the documents that match the query filter.
For example, out of 1000 documents from the stage 1 you select only few documents
In your example, find
First, you are filtering the documents that match the query filter.
For example, if your collection has 1000 documents, you are returning only the documents that match the query condition.
Here You did not specify the fields to return in the documents that match the query filter. Therefore the returned documents will have all fields.
You can use projection in find, instead of using aggregation
db.getCollection('stock_records').find(
{
'info.store_id': '563dcf3465512285781608802a',
'info.created_date': {
$gt: ISODate('2021-07-18T21:07:42.313+00:00')
}
},
{
"info.created_date": 1,
"info.store_id": 1,
"info.store_name": 1,
"_id": 1
}
)
I don't want to expose the full object ID to the client, instead I want to show him only a short of the last 4 chars of the actual object ID of an entity in the collection.
For example: ObjectId("5fcca5d997239a74da0d67a9") will become just 67a9
So it will be much easier to "talk" with ids of documents instead of the full object it.
Then I need to find the document in the DB using only the 67a9.
Is this possible and how?
According to this issue in Jira the resolution is "Won't fix".
ObjectId is not a String, is another object, so $regex is no possible.
Check this example where $regex works ok when _id is an String but not an ObjectId.
So one possible option is duplicate every field _id in another field called id or whatever where the id is in String format.
Then, you can do this query:
db.collection.find({
"_id": {
"$regex": "67a9$"
}
})
Example here where I've added more _id fields that not match the pattern
As pointed out, regex won't work on an ObjectId. But there is an easy workaround. Just use aggregation to first convert your ObjectId into a string and then match it.
db.collection.aggregate([
{
$addFields: {
tempId: { $toString: '$_id' },
}
},
{
$match: {
tempId: { $regex: "67a9"}
}
}
])
Obviously not a great solution to use on very large collections.
{
questions: {
q1: "",
}
}
result after updating:
{
questions: {
q1: "",
q2: ""
}
}
I want to add q2 inside questions, without overwritting what's already inside it (q1).
One solution I found is to get the whole document, and modify it on my backend, and send the whole document to replace the current one. But it seems really in-effiecient as I have other fields in the document as well.
Is there a query that does it more efficiently? I looked at the mongodb docs, didn't seem to find a query that does it.
As turivishal said, you can use $set like this
But you also can use $addFields in this way:
db.collection.aggregate([
{
"$match": {
"questions.q1": "q1"
}
},
{
"$addFields": {
"questions.q2": "q2"
}
}
])
Example here
Also, reading your comment where you say Cannot create field 'q2' in element {questions: "q1"}. It seems your original schema is not the same you have into your DB.
Your schema says that questions is an object with field q1. But your error says that questions is the field and q1 the value.
Suppose to have a collection of MongoDB documents with the following structure:
{
id_str: "some_value",
text: "some_text",
some_field: "some_other_value"
}
I would like to filter such documents so as to obtain the ones with distinct text values.
I learned from the MongoDB documentation how to extract unique field values from a collection, using the distinct operation. Thus, by performing the following query:
db.myCollection.distinct("text")
I would obtain an array containing the distinct text values:
["first_distinct_text", "second_distinct_text",...]
However, this is not the result that i would like to obtain. Instead, I would like to have the following:
{ "id_str": "a_sample_of_id_having_first_distinct_text",
"text": "first_distinct_text"}
{ "id_str": "a_sample_of_id_having_second_distinct_text",
"text": "second_distinct_text"}
I am not sure if this can be done with a single query.
I found a similar question which, however, do not solve fully my problem.
Do you have any hint on how to solve this problem?
Thanks.
You should look into making an aggregate query using the $group stage, and probably using the $first operator.
Maybe something along the lines of:
db.myCollection.aggregate([{ $group : { _id : { text: "$text"},
text: { $first: "$id_str" }
}
}])
try:
db.myCollection.aggregate({$group: {_id: {'text': "$text", 'id_str': '$id_str'}}})
More information here: http://docs.mongodb.org/manual/reference/method/db.collection.aggregate/
I have a mongoDB document that has the following structure:
{
user:user_name,
streams:[
{user:user_a, name:name_a},
{user:user_b, name:name_b},
{user:user_c, name:name_c}
]
}
I want to use $pullAll to remove from the streams array, passing it an array of streams (the size of the array varies from 1 to N):
var streamsA = [{user:"user_a", name:"name_a"},{user:"user_b", name:"name_b"}]
var streamsB = [{name:"name_a", user:"user_a"},{name:"name_b", user:"user_b"}]
I use the following mongoDB command to perform the update operation:
db.streams.update({name:"user_name", {"$pullAll:{streams:streamsA}})
db.streams.update({name:"user_name", {"$pullAll:{streams:streamsB}})
Removing streamsA succeeds, whereas removing streamsB fails. After digging through the mongoDB manuals, I saw that the order of fields in streamsA and streamsB records has to match the order of fields in the database. For streamsB the order does not match, that's why it was not removed.
I can reorder the streams to the database document order prior to performing an update operation, but is there an easier and cleaner way to do this? Is there some flag that can be set to update and/or pullAll to ignore the order?
Thank You,
Gary
The $pullAll operator is really a "special case" that was mostly intended for single "scalar" array elements and not for sub-documents in the way you are using it.
Instead use $pull which will inspect each element and use an $or condition for the document lists:
db.streams.update(
{ "user": "user_name" },
{ "$pull": { "streams": { "$or": streamsB } }}
)
That way it does not matter which order the fields are in or indeed look for an "exact match" as the current $pullAll operation is actually doing.